substrate is rate limiting (although a preceding intramolecular
rearrangement of the insertion product to the secondary amide
cannot be ruled out a priori).17
Conclusions
It has been shown that both neutral and cationic Group 3 metal
alkyl species, with two different types of monoanionic ancillary lig-
ands, can catalyse the intramolecular hydroamination/cyclisation
of the standard substrate 2,2-dimethyl-4-pentenylamine. For the
4-electron bidentate amidinate ligand, the neutral catalysts are
considerably more active than their cationic counterparts, whereas
for the 8-electron tetradentate triamine–amide ligands the reverse
is the case. It appears that both the availability of sufficient
room in the coordination sphere of the metal and the strength
of the metal–amide bond play a role in determining the catalyst
effectiveness.
From the data obtained in this study it appears that, with
respect to absolute activity, cationic rare earth metal catalysts
are unlikely to better the rates in hydroamination/cyclisation
that can be achieved with neutral catalysts. Nevertheless, the
generation of cationic species may provide a means to achieve
meaningful activities with sterically demanding ancillary ligands.
This approach could be useful with asymmetric ligands that
aim for a high enantioselectivity in this reaction. Up until
now, this has been approached mainly by using dianionic
ancillary ligands (linked cyclopentadienyls,18 bisphenolates19).
Using cationic active species, families of sterically demanding
asymmetric monoanionic multidentate ligands (e.g. derivatives of
the bis(oxazoline)methyenyl ligands as employed by Marks and
coworkers20) could be applied to this reaction with improved
reaction rates. In this case, best activities may be expected with
ligands that employ strongly donating moieties to weaken the
reactive metal–amide bond.
Discussion
As is evident from the results presented above, the relative catalytic
activity for the hydroamination/cyclisation reaction of cationic
monalkyl versus neutral dialkyl Group 3 metal species is highly
dependent on the ancillary ligand system. For the coordinatively
relatively unencumbering dihapto amidinate ligand, the catalytic
activity of the cationic species is over two orders of magnitude less
than that of the neutral analogue. For the coordinatevely demand-
ing tetrahapto triamine–amide ligands, the cationic derivatives are
clearly more active than the neutral species.
When directly comparing the activities of the cationic catalysts
+
+
{[A]YR(THF)} versus {[Me2B]YR} (which is possible, as both
show zero order dependence on the substrate concentration, at
least at <50% conversion), the more electron-rich triamino-amide
catalyst is 22 times more active than the amidinate catalyst. As-
suming that the zero order dependence on substrate concentration
is an indication that the intramolecular insertion of the olefinic
group into the metal–amide bond is rate-determining (as appears
to be the case with most hydroamination-cyclisation catalysts),
this difference could be related to the relative strength of the Y–
amide bonds. This bond is expected to be stronger for the more
electron deficient species, the amidinate complex, which is also the
less active catalyst.
The tetradentate triamine–amide ligands occupy a significantly
larger part of the coordination sphere of the metal, and also
impart more electron density to the metal centre, than the
bidentate amidinate ligand. It is likely that the former systems
benefit from the creation of a vacant site by the removal of
one of the alkyl groups from the metal centre. Nevertheless, the
first order dependence on substrate concentration of the more
sterically encumbered systems, with the iPr2B and C ligands,
indicates that here the product/substrate exchange step is likely
to be rate limiting.17 This indicates that direct structure–property
relationships cannot always be drawn in a straightforward manner,
and that the availability of kinetic data is necessary for a true
comparison of catalysts.
If the suggestion that the relative strength of the metal–amide
bonds is the determining factor for the activity of Group 3 metal
catalysts for hydroamination/cyclisation is correct (at least for
those catalysts that show zero order substrate dependence), it
is unlikely that cationic Group 3 metal catalysts will be able to
improve in an absolute sense on the high activities that can be
obtained with neutral catalysts. An increase in activity upon going
from a neutral to a cationic catalyst can then only be expected when
the neutral catalyst is relatively slow due to steric or coordinative
encroachment of the metal centre and where the creation of an
additional free coordination site will relieve this. Although no
kinetic data are available for that system, it is likely that the
increased catalyst activity of the cationic scandium b-diketiminato
catalyst (with 2,6-diisopropylphenyl substituents on the nitrogen
atoms) relative to the neutral analogue12 is due to the relief of steric
congestion around the metal centre. For the scandium amidinate
catalysts, with a ligand that is related but has a significantly smaller
bite-angle, the neutral catalyst is already by far the more efficient.
Acknowledgements
The authors thank D. J. Beetstra for useful discussions. One of the
authors (H.T.) expresses his special thanks to the centre of excel-
lence (21COE) program “Creation of Integrated EcoChemistry”
of Osaka University.
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